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Introducing a New Azoaromatic Pincer Ligand. Isolation and Characterization of Redox Events in Its Ferrous Complexes

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journal contribution
posted on 05.05.2014, 00:00 by Pradip Ghosh, Subhas Samanta, Suman K. Roy, Serhiy Demeshko, Franc Meyer, Sreebrata Goswami
The isolation and complete characterization of a new bis-azoaromatic ligand, 2,6-bis­(phenylazo)­pyridine (L), are described, and its coordination to iron­(II) is reported. A pseudo-trigonal-bipyramidal mixed-ligand complex of iron­(II), FeLCl2 (1), and a homoleptic octahedral iron complex, mer-[Fe­(L)2]­ClO4 [2]­ClO4, have been synthesized from L and FeCl2 or hydrated Fe­(ClO4)2, respectively, in boiling methanol. Determination of the X-ray crystallographic structure together with magnetic data (≈ 5.06 μB) and Mössbauer analysis of 1 established a high-spin Fe­(II) complex ligated by one neutral 2,6-bis­(phenylazo)­pyridine ligand. The X-ray crystallographic structure (showing dN–N > 1.30 Å), Mössbauer data, and magnetic susceptibility measurements (≈ 1.65 μB) as well as a nearly isotropic EPR signal with only a small metal contribution at g = 1.968, on the other hand, suggest a low-spin Fe­(II) complex with a one-electron-reduced radical ligand coordination in [2]­ClO4. The ligand and the metal complexes have well-behaved redox properties, with the ligand(s) functioning as the redox-active site(s) responsible for redox events. The uncoordinated ligand, L, displays a reversible one-electron wave at −1.07 V and a quasi-reversible wave at −1.39 V. The partially reduced ligand L•– shows a single-line EPR spectrum at g = 2.001, signifying that L•– is a free radical. While complex 1 shows a reversible reduction at −0.08 V and an irreversible cathodic response at −0.98 V, the bis-chelate [2]­ClO4 undergoes a reversible one-electron oxidation at 0.54 V and three successive reversible one-electron reductions at −0.18, −0.88, and −1.2 V, all occurring at the ligands without affecting the metal ion oxidation state. The electronic structures of the parent monocationic complex [2]+ and its oxidized and reduced forms, generated by exhaustive electrolyses, have been characterized by using a host of spectroscopic techniques and density functional theory (DFT). It is found that the 2,6-bis­(phenylazo)­pyridine ligand (L) is truly redox noninnocent and is capable of coordinating transition-metal centers in its neutral ([L]0), monoanionic monoradical ([L]), and dianionic diradical ([L••]2–) forms.